Depending on the medical field, microscopes used during surgery must meet different requirements insofar as the illumination of the operating area is concerned. The type of illumination is determined substantially by the angle formed by the axis of the illumination beam and the axis of the microscope lens.
In microsurgery in non-ophthalmic fields, e.g., otolaryngological surgery and neurosurgery, the operating area is illuminated obliquely with light directed along a path close to the lens axis. This is referred to in the art as "coaxial illumination", and it reduces the diameter of the illuminated area to so-called spot illumination, which can be directed into narrow, deep body cavities.
However, for microsurgical procedures on the eye, some microscopes have projected the illumination vertically, i.e., along the optical axis of the microscope lens, onto the operating area. With this latter type of illumination, known as "0.degree. illumination", the vertically impinging light rays are diffusely reflected by the retina, and this reflected illumination makes the capsule of the lens (i.e., the shell of the lens of the eye) appear in reddish transmitted light. As a result of this, tissue remainders which must be removed by suction from the eye lens are rendered visible by high contrast.
Nonetheless, such vertically impinging light is required only during a portion of the ophthalmic surgical procedure or as partial illumination, so there is often a need to illuminate the patient's eye with either or both vertical and oblique impinging light during ophthalmic surgery.
The present invention meets this need with a special system for surgical microscopes which can provide either or both vertical and oblique illumination in combinations selected by the operator.